U.S. patent number 8,951,638 [Application Number 13/894,533] was granted by the patent office on 2015-02-10 for silicon carbide powder for producing silicon carbide single crystal and a method for producing the same.
This patent grant is currently assigned to Denki Kagaku Kogyo Kabushiki Kaisha, National Institute of Advanced Industrial Science and Technology. The grantee listed for this patent is Denki Kagaku Kogyo Kabushiki Kaisha, National Institute of Advanced Industrial Science and Technology. Invention is credited to Tomohisa Katou, Hiroshi Murata, Yusuke Takeda.
United States Patent |
8,951,638 |
Katou , et al. |
February 10, 2015 |
Silicon carbide powder for producing silicon carbide single crystal
and a method for producing the same
Abstract
A silicon carbide powder for the production of a silicon carbide
single crystal has an average particle diameter of 100 .mu.m or
more and 700 .mu.m or less and a specific surface area of 0.05
m.sup.2/g or more and 0.30 m.sup.2/g or less. A method for
producing a silicon carbide powder for the production of the
silicon carbide single crystal including sintering a silicon
carbide powder having an average particle diameter of 20 .mu.m or
less under pressure of 70 MPa or less at a temperature of
1900.degree. C. or more and 2400.degree. C. or less and in a
non-oxidizing atmosphere, thereby obtaining a sintered body having
a density of 1.29 g/cm.sup.3 or more; adjusting particle size by
means of pulverization of the sintered body; and removing
impurities by means of an acid treatment.
Inventors: |
Katou; Tomohisa (Tsukuba,
JP), Takeda; Yusuke (Machida, JP), Murata;
Hiroshi (Machida, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
National Institute of Advanced Industrial Science and
Technology
Denki Kagaku Kogyo Kabushiki Kaisha |
Chiyoda-ku, Tokyo
Chuo-ku, Tokyo |
N/A
N/A |
JP
JP |
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Assignee: |
Denki Kagaku Kogyo Kabushiki
Kaisha (Tokyo, JP)
National Institute of Advanced Industrial Science and
Technology (Tokyo, JP)
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Family
ID: |
46083944 |
Appl.
No.: |
13/894,533 |
Filed: |
May 15, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20130266810 A1 |
Oct 10, 2013 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCT/JP2011/075930 |
Nov 10, 2011 |
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Foreign Application Priority Data
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Nov 15, 2010 [JP] |
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2010-254378 |
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Current U.S.
Class: |
428/402; 264/676;
264/682; 423/345; 501/91; 501/90; 501/89; 423/346; 501/88 |
Current CPC
Class: |
C30B
29/36 (20130101); C01B 32/984 (20170801); C30B
35/007 (20130101); C30B 23/02 (20130101); C01B
32/956 (20170801); Y10T 428/2982 (20150115) |
Current International
Class: |
B32B
5/16 (20060101) |
Field of
Search: |
;428/402 ;423/345,346
;501/88-91 ;264/676,682 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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101746758 |
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Jun 2010 |
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CN |
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H0354111 |
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Mar 1991 |
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JP |
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H04270105 |
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Sep 1992 |
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JP |
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H0948605 |
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Feb 1997 |
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JP |
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2005-239496 |
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Sep 2005 |
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JP |
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2009-173501 |
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Aug 2009 |
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JP |
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2009-173501 |
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Aug 2009 |
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JP |
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2009173501 |
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Aug 2009 |
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JP |
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Other References
International Search Report of PCT/JP2011/075930 mailed Feb. 21,
2012. cited by applicant .
Yu.M. Tairov and V.F. Tsvetkov, "General Principles of Growing
Large-Size Single Crystals of Various Silicon Carbide Polytypes",
Journal of Crystal Growth, 1981, vol. 52, pp. 146-150. cited by
applicant .
Japanese Office Action for application No. 2010-254378 dated Oct.
7, 2014. cited by applicant .
Chinese Office Action for application No. 201180055027.5 dated Oct.
22, 2014 cited by applicant.
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Primary Examiner: Kiliman; Leszek
Attorney, Agent or Firm: Lowe Hauptman & Ham, LLP
Claims
The invention claimed is:
1. A silicon carbide powder for producing a silicon carbide single
crystal comprising: an average particle diameter of 100 .mu.m or
more and 700 .mu.m or less and a specific surface area of 0.05
m.sup.2/g or more and 0.30 m.sup.2/g or less, wherein the silicon
carbide powder has a particle state in which primary particles
having an average particle diameter of 5 .mu.m or more and 200
.mu.m or less are sintered.
2. A method for producing a silicon carbide powder for the
production of the silicon carbide single crystal according to claim
1 comprising: sintering a silicon carbide powder having an average
particle diameter of 20 .mu.m or less under pressure of 70 MPa or
less at a temperature of 1900.degree. C. or more and 2400.degree.
C. or less and in a non-oxidizing atmosphere, thereby obtaining a
sintered body having a density of 1.29 g/cm.sup.3 or more;
adjusting particle size by means of pulverization of the sintered
body; and removing impurities by means of an acid treatment.
3. A method for producing a silicon carbide powder for the
production of a silicon carbide single crystal according to claim
1, wherein the silicon carbide powder is obtained by heating and
sintering a raw material mixed product at a temperature of
1800.degree. C. or more and 2300.degree. C. or less under a
non-oxidizing atmosphere condition, the raw material mixed product
includes a silicon source, a carbon source and 5% by mass or more
and 50% by mass or less of a silicon carbide powder with average
particle diameter of 20 .mu.m or more.
4. A method for producing a silicon carbide powder for the
production of a silicon carbide single crystal according to claim
1, comprising: producing the silicon carbide powder by heating and
sintering metal silicon and a carbon source with an average
particle diameter of 100 .mu.m or more and 700 .mu.m or less under
a non-oxidizing atmosphere condition at a temperature of
1300.degree. C. or more and 1400.degree. C. or less; and
post-processing the silicon carbide powder by heating at a
temperature of 1800.degree. C. or more and 2300.degree. C. or less
under a non-oxidizing atmosphere condition.
5. The method for producing a silicon carbide powder for the
production of a silicon carbide single crystal according to claim
4, wherein the carbon source is an organic compound and the organic
compound is carbonized by heating at a temperature of 500.degree.
C. or more and 1000.degree. C. or less under a non-oxidizing
atmosphere condition before producing the silicon carbide powder by
heating and sintering.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority
from the prior Japanese Patent Application No. 2010-254378, filed
on Nov. 15, 2010 and PCT International Application
PCT/JP2011/075930, filed on Nov. 10, 2011, the entire contents of
which are incorporated herein by reference.
FIELD
The present invention is related to a silicon carbide powder for
producing silicon carbide single crystal and a method for producing
the silicon carbide powder.
BACKGROUND
Conventionally, a sublimation recrystallization method (Modified
Lely Method) in which silicon carbide powder which is a raw
material is sublimed under high temperature conditions of
2000.degree. C. or more and a single crystal is grown on a silicon
carbide seed crystal is known as a method of producing silicon
carbide powder single crystal (Yu. M Tairov and V. F. Tsvetkov,
Journal of Crystal Growth vol. 52 (1981) pp. 146-150).
In addition, it is also known that many crystal defects are
produced by mixing impurities within a single crystal in the case
where silicon carbide powder including a large amount of impurities
is used in a sublimation recrystallization method.
The Acheson process and chemical vapor deposition method are known
as methods of producing silicon carbide powder. However, there is a
problem of impurities when the silicon carbide powder is obtained
using the Acheson method and low productivity when the silicon
carbide powder is obtained using a chemical vapor deposition
method.
In addition, a method of producing silicon carbide powder for
producing silicon carbide single crystal is disclosed (patent
document 1) in which a mixed product of a liquid shaped silicon
compound and an organic compound which produces carbon by heating
is heated and reacted and the contained amount of each impurity
element is 0.5 ppm or less.
In addition, silicon carbide powder for producing silicon carbide
single crystal is required to have a relatively large average
particle diameter in order to maintain a stable sublimation rate
under a single crystal growth condition. For example, an average
particle diameter of 10.about.500 .mu.m is disclosed in Japanese
Laid Open Patent H9-48605.
SUMMARY
The present invention provides a silicon carbide powder which
displays a high and stable sublimation rate during single crystal
growth by means of a sublimation recrystallization method; and a
method for producing the silicon carbide powder.
The present invention adopts the following means in order to solve
the problems described above. (1) A silicon carbide powder for
producing a silicon carbide single crystal including an average
particle diameter of 100 .mu.m or more and 700 .mu.m or less and a
specific surface area of 0.05 m.sup.2/g or more and 0.30 m.sup.2/g
or less. (2) The silicon carbide powder for producing a silicon
carbide single crystal according to (1) wherein primary particles
having an average particle diameter of 5 .mu.m or more and 200
.mu.m or less are sintered. (3) A method for producing a silicon
carbide powder for the production of the silicon carbide single
crystal according to (1) further including sintering a silicon
carbide powder having an average particle diameter of 20 .mu.m or
less under pressure of 70 MPa or less at a temperature of
1900.degree. C. or more and 2400.degree. C. or less and in a
non-oxidizing atmosphere, thereby obtaining a sintered body having
a density of 1.29 g/cm.sup.3 or more; adjusting particle size by
means of pulverization of the sintered body; and removing
impurities by means of an acid treatment. (4) The method for
producing a silicon carbide powder for the production of a silicon
carbide single crystal according to (1) wherein the silicon carbide
powder is obtained by heating and sintering a raw material mixed
product at a temperature of 1800.degree. C. or more and
2300.degree. C. or less under a non-oxidizing atmosphere condition
using 5% by mass or more and 50% by mass or less of a silicon
carbide powder with average particle diameter of 20 .mu.m or more
with a silicon source and a carbon source average particle diameter
of 20 .mu.m or more. (5) The method for producing a silicon carbide
powder for the production of a silicon carbide single crystal
according to (1) further including producing the silicon carbide
powder by heating and sintering metal silicon and a carbon source
with an average particle diameter of 100 .mu.m or more and 700
.mu.m or less under a non-oxidizing atmosphere condition at a
temperature of 1300.degree. C. or more and 1400.degree. C. or less;
and post-processing the silicon carbide powder by heating at a
temperature of 1800.degree. C. or more and 2300.degree. C. or less
under a non-oxidizing atmosphere condition. (6) The method for
producing a silicon carbide powder for the production of a silicon
carbide single crystal according to (5) wherein the carbon source
is an organic compound and the organic compound is carbonized by
heating at a temperature of 500.degree. C. or more and 1000.degree.
C. or less under a non-oxidizing atmosphere condition before
producing the silicon carbide powder heating and sintering.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the characteristics of a silicon carbide powder
related to one example of the present invention;
FIG. 2 shows the characteristics of a silicon carbide powder
related to one embodiment of the present invention;
FIG. 3 shows the characteristics of a silicon carbide powder
related to one embodiment of the present invention; and
FIG. 4 shows the growth of a silicon carbide single crystal related
to one embodiment of the present invention.
DESCRIPTION OF EMBODIMENTS
A high and stable sublimation rate is demanded as a characteristic
of a silicon carbide powder for producing a silicon carbide single
crystal. For the reasons described above, the silicon carbide
powder of the present invention has an average particle diameter of
100 .mu.m or more and 700 .mu.m or less and a specific surface area
of 0.05 m.sup.2/g or more and 0.30 m.sup.2/g or less. In the case
where the average particle diameter is less than 100 .mu.m or the
specific surface area exceeds 0.30 m.sup.2/g, the sublimation rate
(2000.degree. C. or more and 2500.degree. C. or less) when
producing a silicon carbide single crystal is high during the
sublimation initial stage. However, the specific surface area
decreases as sintering of the silicon carbide powder gradually
progresses and the sublimation rate drops. On the other hand, in
the case where the average particle diameter exceeds 700 .mu.m or
the specific surface area is less than 0.05 m.sup.2/g, the specific
surface area of the silicon carbide itself decreases and this
reduces the sublimation rate which is not desirable.
In addition, the silicon carbide powder of the present invention
has a particle state in which pairs of primary particles with
particle diameter of 5 .mu.m or more and 200 .mu.m or less are
sintered. If the primary particle diameter of the silicon carbide
powder is less than 5 .mu.m or exceeds 200 .mu.m, the strength of
the silicon carbide powder decreases and it is not possible to
maintain the particle state when handling which is not
desirable.
Furthermore, because the silicon carbide powder of the present
invention has a contained amount of each impurity of 1 ppm or less,
crystal defects are few and it is possible to use the silicon
carbide powder as a raw material of a silicon carbide single
crystal with excellent conductivity control. Here, an impurity
element is an element belongs to group 1 to group 16 and in the
periodic table of the 1989 IUPAC Inorganic Chemical Nomenclature
Revised Edition and has an atomic number of three or more except
atomic numbers 6.about.8 and 14. In the present invention, the
contained amount of Li, B, Na, Mg, Al, P, K, Ca, Ti, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, Zr, Mo, Pd, Cd, Sb, Ba and W is measured.
A number of producing methods of the silicon carbide powder of the
present invention are exemplified.
(Sintered Powder Crushing Method)
A producing method wherein a silicon carbide sintered body is
produced by sintering the silicon carbide powder, the particle size
is adjusted by crushing the obtained silicon carbide sintered body
and impurities mixed in during the crushing process are removed
using an acid treatment can be given as an example of a first
producing method of the silicon carbide powder of the present
invention. This is described in detail below.
Either an .alpha. type silicon carbide or .beta. type silicon
carbide can be used as the silicon carbide used in the sintered
body creation process. High grade purification is easy if a high
purity silicon carbide powder is used and reducing the contained
amount of each impurity to a few ppm of less is preferred. An
average particle diameter of 20 .mu.m or less is preferred and 10
.mu.m is desirable considering sintering performance. When average
particle diameter exceeds 20 .mu.m, the contact area between
adjacent powder particles decreases and sintering becomes
insufficient. In addition, because the surface amount of oxygen
increases when the particle diameter is small causing a decrease in
sintering performance, the lower limit value of particle diameter
is preferred to be 0.1 .mu.m or more and more preferably 0.4 .mu.m
or more.
In addition, in a conventional producing method, a metal auxiliary
agent is added in the sintering of the silicon carbide with the aim
of densification. However, in the present invention, it is
preferred that the silicon carbide is sintered without using a
metal auxiliary agent from the view point of reducing
impurities.
The above described silicon carbide powder is filled into a metal
mold and a compact body is created. The compact metal mold which is
used here is preferred to be a metal mold having a part or all of
the mold being graphite so that the compact and the metal sections
of the metal mold do not contact considering the purity of the
sintered body to be obtained. The compact which is obtained is
pressure sintered under a non-oxidizing atmosphere and a sintered
body is created. A hot press sintering method is available as the
pressure sintering method.
In a hot press sintering method the sintering temperature is
1900.degree. C. or more and 2400.degree. C. or less and more
preferably 2000.degree. C. or more and 2200.degree. C. or less. If
the sintering temperature is less than 1900.degree. C., the density
of the sintered body is insufficient and if the sintering
temperature exceeds 2400.degree. C., the silicon carbide raw
material begins to sublime which is not desirable. The applied
pressure is 70 MPa or less. In the case where no pressure is
applied or the applied pressure is low, the density of the sintered
body to be obtained is low and therefore an applied pressure of 10
MPa or more is preferred. In addition, if the applied pressure
exceeds 70 MPa, the hot press fixtures such as a dice or punch etc
can become damaged which is not desirable considering producing
efficiency.
The heating time during the sintering process is selected from a
time period according to type of silicon carbide powder which is
filled or the density of the sintered body to be obtained and a
maximum temperature is preferred to be maintained in a range of 1
hour or more and 4 hours or less.
The obtained silicon carbide sintered body is crushed using a
crusher such as a hammer mill, a bantam mill or a jaw crusher for
example and a silicon carbide powder with an average particle
diameter of 100 .mu.m or more and 700 .mu.m or less is obtained by
separation using a sieve. It is possible to use acid cleaning such
as hydrochloric acid or hydrofluoric acid or a mixed acid of
hydrofluoric acid and nitric acid to remove the impurities that are
mixed in during the crushing process.
The density of the silicon carbide sintering is 1.29 g/cm.sup.3
(relative density 40%) or more. Here, relative density is a value
calculated with a theoretical density of 3.22 g/cm.sup.3 given as
100%. If the relative density exceeds 40%, mechanical strength of
the sintered body decreases, the occurrence of fine particles in
the following crushing process increases and productivity decreases
which is not desirable. In addition, although it is possible to use
a densified sintered body, in the case where a porous sintered body
(relative density of 40% or more) with a low density is crushed, it
is possible to obtain a powder with a higher specific surface area.
From this view point, the relative density of the sintered body of
the present invention is preferred to be 60% or more and 80% or
less.
(Nucleus Addition Method)
A method of heating and sintering a source material mixed product
of silicon carbide powder used as the silicon source and carbon
source can be given as an example of a second producing method of
the silicon carbide powder of the present invention. This is
explained in detail below.
Metal silicon, silicon nitride, silicon oxide, liquid shaped
silicon compound (for example, tetraalkoxysilane or its polymer)
and graphite powder, carbon black, or an organic compound with
remaining carbon due to heating (for example, phenol resin or fran
resin) are available as the as the silicon and carbon sources for
producing a silicon carbide powder. Metal silicon/carbon black,
silicon nitride/carbon black are preferred as a combination of raw
materials considering handling and purity of the silicon carbide
powder after heating and sintering.
In the case of using metal silicon/carbon black, silicon
nitride/carbon black, the mol ratio (C/Si ratio) of the silicon and
carbon source is preferred to be 0.8 or more and 1.5 or less and
more preferably 0.9 or more and 1.1 or less. When the C/Si ratio is
less than 0.8, non-reacted Si often remains and when the C/Si ratio
exceeds 1.5, non-reacted C often remains and a removal process is
required.
Silicon carbide powder used as a raw material is preferred to have
an average particle diameter of 20 .mu.m or more. In addition,
silicon carbide powder of a source material mixed product is
preferred to be 5% by mass or more and 50% by mass or less. A
source material mixed product is heated and sintered under a
non-oxidizing atmosphere.
The silicon carbide powder used as a raw material selectively
becomes the start point for particle formation when heated and
sintered and large secondary particles are formed when sintering is
promoted. In the case where average particle diameter of the
silicon carbide to be added is less than 20 .mu.m, the particle
diameter of the silicon carbide powder which is obtained by
sintering decreases which is not desirable. Although there is not
upper limit to the average particle diameter of the silicon carbide
to be added, because coarsening effects due to particle growth and
sintering are not significantly shown when the particle diameter is
too large, the average particle diameter is preferred to be 200
.mu.m or less.
While .alpha. silicon carbide or .beta. silicon carbide can be used
as the silicon carbide powder used as a raw material, because
.beta. silicon carbide is produced in the producing method of the
present invention, the use of .beta. silicon carbide powder is
preferred. Furthermore, using .beta. silicon carbide powder
producing from the same raw material as the silicon source and
carbon source used to produce the silicon carbide powder is more
preferable.
The silicon carbide powder used as a raw material within a raw
material mixed product is preferred to be 5% by mass or more and
50% by mass or less. In the case where the silicon carbide powder
is less than 5% by mass, formation and particle growth of the
silicon carbide particles occurs anew in parts other than the
silicon carbide powder, sintering progresses and secondary
particles do not achieve a sufficient particle diameter, and when
the silicon carbide powder is more than 50% by mass, the starting
points of particle growth increase and particle growth effects
decrease which is not desirable.
In the present invention, a source material mixed product added
with silicon carbide powder is preferred to be heated and sintered
at a temperature of 1800.degree. C. or more and 2300.degree. C. or
less and more preferably 1900.degree. C. or more and 2100.degree.
C. or less. If the heating temperature is less than 1800.degree.
C., particle growth and sintering of particle pairs are
insufficient and when the heating temperature exceeds 2300.degree.
C., sublimation of the silicon carbide which is produced begins
which is not desirable.
In addition, although the heating time during heating and sintering
is selected from a time range so that non-reactive objects do not
exist and where particle growth and sintering of particle pairs
progresses, the time period at a maximum temperature is preferred
to be maintained in a range of 1.about.10 hours.
By performing a separation process of the silicon carbide powder
which is obtained by the method described above using a sieve for
example, a silicon carbide powder with an average particle diameter
of 100 .mu.m or more and 700 .mu.m or less is obtained.
Furthermore, by repeating the above described process a further one
time or more using the silicon carbide powder which is obtained, it
is possible to obtained a coarser silicon carbide powder.
(Solid-Phase Reaction Method)
Uniformly covering an organic compound with remaining carbon which
becomes a carbon source due to heating, on a metal silicon which is
a silicon source, heating and sintering at a metal silicon melting
point temperature (1410.degree. C.) or less under a non-oxidizing
atmosphere to obtain a silicon carbide powder and heating the
obtained silicon carbide powder at a higher temperature than that
described above under a non-oxidizing atmosphere is an example of a
third producing method of the silicon carbide powder of the present
invention. This is described in detail below.
Considering the purity of the silicon carbide powder after heating
and sintering, the metal silicon used as a raw material is
preferred to have a high grade purity and the contained amount of
each impurity is preferred to be reduced to a few ppm or less. In
addition, considering the particle diameter of the silicon carbide
powder which is produced as a result of heating and sintering, the
particle diameter of the metal silicon is preferred to be 100 .mu.m
or more and 700 .mu.m or less. In addition, in the case of using a
metal silicon having a particle diameter which exceeds 700 .mu.m, a
sintering reaction does not proceed efficiently, non-reacted metal
silicon still remains and the reaction rate drops. When the
particle diameter is less than 100 .mu.m, a silicon carbide powder
with an average particle diameter of 100 .mu.m or more and 700
.mu.m or less and a specific surface area of 0.05 m.sup.2/g or more
and 0.30 m.sup.2/g or less cannot be obtained which is not
desirable.
While organic compounds with residual carbon due to heating are
available to be used as the carbon source, it is also possible to
use resins such as a phenol resin, fran resin, epoxy resin and
xylene resin etc. While a liquid shaped material at normal
temperature or a softening or liquid shaped material due to heating
such as thermoplasticity or thermal melting are mainly used, among
these a resol type phenol resin is favorable considering the
purpose of covering a metal silicon.
When uniformly mixing a metal silicon and carbon source, a solid
material formed by curing a mixed product of the metal silicon and
carbon source is preferred. For example, in the case where a liquid
shaped carbon source is used, a mixed product of the metal silicon
and carbon source is cured and following this the silicon carbide
is sintered. Examples of the curing method are a cross linking
method by heating and a curing method by a curing catalyst. In the
case where the carbon source is a phenol resin, acids such as
toluenesulfonic acid, maleic acid, maleic acid anhydride and
hydrochloric acid can be used as a curing catalyst.
In the producing method of the present invention, it is preferred
that a carbonizing process is performed for heating a mixed product
of a metal silicon and carbon source in advance under a
non-oxidizing atmosphere. When a heating carbonizing process is
performed, it is possible to select a heating temperature according
to the type of organic compound of the carbon source. However, a
heating temperature of 500.degree. C. or more and 1000.degree. C.
or less is preferred. In addition, a heating time of 30 minutes or
more and 2 hours or less is preferred. If the heating time is less
than 30 minutes, the carbonizing process is insufficient and
improvements in the effects of the invention are not observed if
heating is performed for over 2 hours. In addition, it is possible
to use nitrogen or argon etc in the on-oxidizing atmosphere.
In the producing, method of the present invention, the mole ratio
(C/Si) between silicon and carbon source of a mixed product of a
metal silicon and carbon source is defined by a carbon product
intermediate obtained by carbonizing from the raw material mixed
product and its value is preferred to be 0.8 or more and 1.5 or
less and more preferably 0.9 or more and 1.1 or less. In the case
where the C/Si is less than 0.8, non-reacted Si remains and the
reaction rate decreases and in the case where the C/Si exceeds 1.5,
a large amount of non-reacted C remains and a removal process
becomes necessary which is not desirable.
Silicon carbide powder is produced by heating and sintering a raw
material mixed product obtained by a carbonizing process. In the
sintering process, sintering is performed at a temperature for
maintaining the state of the metal silicon within the raw material
mixed product, that is, at the melting point of the metal silicon
(1410.degree. C.) or less. In this way, it is possible to obtain a
silicon carbide powder having a particle diameter whereby the
particle diameter of the metal silicon source is maintained. The
heating temperature is preferred to be 1300.degree. C. or more and
1400.degree. C. or less. In addition, if the heating temperature is
less than 1300.degree. C., non-reacted metal silicon easily remains
which is not desirable.
Because the silicon carbide powder in the producing method of the
present invention is obtained via a silicon carbide reaction due to
a solid-phase-solid-phase reaction, the reaction rate is slow and
the maximum heating temperature is preferred to be in a range of 4
hours or more and 30 hours or less that non-reactive objects do not
exist.
The silicon carbide powder to be obtained is a powder in which nano
to sub-micron sized primary particles are sintered and secondary
particles becomes a powder where the particle form of the metal
silicon which is the raw material is maintained. That is, it is
possible to control the particle diameter of the silicon carbide
powder obtained from the particle diameter of the raw material
metal silicon.
In order to grow the primary particles of the obtained silicon
carbide powder, heating is performed at a higher temperature than
the silicon carbide formation temperature and maintained at this
temperature as a post process. The heating temperature is preferred
to be 1800.degree. C. or more and 2300.degree. C. or less and more
preferably 1900.degree. C. or more and 2100.degree. C. or less. If
the heating temperature is less than 1800.degree. C. or less,
particle growth is insufficient and if the heating temperature
exceeds 2300.degree. C., sublimation of the produced silicon
carbide begins which is not desirable.
The heating time in the post process is preferred to be in a range
where the primary particles of the obtained silicon carbide powder
are sufficiently grown and is preferred to be in range of 1 hour or
more and 10 hours or less and maintained at a maximum
temperature.
In the producing method of the present invention, if the hearing
conditions described above are satisfied then there is no
particular imitation to the producing device and method of
continuous producing. That is, heating and sintering in the silicon
carbide production process and the heating in the post process may
be performed continuously while controlling the heating conditions
in one heating furnace.
By performing a separation process of the obtained silicon carbide
powder using a sieve for example, a silicon carbide powder with an
average particle diameter of 100 .mu.m or more and 700 .mu.m or
less is obtained.
A silicon carbide single crustal can be obtained from the silicon
carbide powder of the present invention using a Modified Lely
Method. In the Modified Lely Method, a seed crystal is placed on a
part of a lid of a graphite container, the silicon carbide powder
of the present invention is filled into the graphite container and
a single crystal is grown by a sublimation recrystallization
method.
EXAMPLES
Examples of the present invention are explained below. In addition,
in the present examples, produce of a silicon carbide single
crystal was attempted using a Modified Lely Method in order to
confirm the effects of the silicon carbide powder of the present
invention.
Creation of Silicon Carbide Powder
(Sintered Powder Crushing Method)
Example 1
The silicon carbide powder which is a raw material used the
following synthesized product.
Metal silicon (silicon sludge, average particle diameter 1.0 .mu.m,
purity 5N produced by Toshiba LSI) and acetylene black (denka
black, average particle diameter 0.04 .mu.m, produced by Denki
Kagaku Kogyo) were weighed to produce a mol ratio (C/Si) of 1.0
between the raw materials silicon and carbon, and the raw material
powder was adjusted after mixing using a mortar in ethyl alcohol
and dried. The raw material powder was put into a graphite
crucible, and the silicon carbide powder was obtained by heating in
a carbon heater under an argon atmosphere at a temperature of
1900.degree. C. for 2 hours. The obtained silicon carbide powder
was crystal phase analyzed using an X-ray diffractometer (MXP-3
produced by Mac Science) and the powder was a .beta. type (3C
phase) silicon carbide.
In addition, a particle diameter distribution measurement was
performed using laser diffraction, scattering method using a
particle size distribution measurement device (LS-230 produced by
Beckman Coulter). Adjustment of the particle diameter distribution
measurement sample was performed according to the measurement
conditions of silicon nitride in table 1 of the attached
explanation JIS R 1629-1997 as a general rule and the refraction
index was 2.6. As a result the average particle diameter was 6.0
.mu.m.
Next, the obtained silicon carbide powder is filled into a graphite
mold with an interior diameter of 50 mm and after preforming, a
silicon carbide sintered body was created by heating at
2200.degree. C. for 2 hours under a pressure of 20 MPa in an argon
atmosphere using a high frequency induction heating type hot press
device. The density of the obtained sintered body was measured
using a dimensions, mass measurement and was measured at 2.02
g/cm.sup.3 (absolute density 62.7%).
The created silicon carbide sintered body was crushed at a rotation
rate of 2400 rpm using a micro-bantam mill (AP-B, produced by
Hosokawamicron). The crushed powder was separated using a sieve
with gaps of 300 .mu.m and 500 .mu.m. As a result of separation,
the yield of powder between 300 .mu.m or more and 500 .mu.m or less
was 82%. Furthermore, the separated powder was heated at 60.degree.
C. in a mixed acid of hydrofluoric acid, nitric acid and distilled
water with a volume ratio of 1:1:1.
After thermolysis was performed on the obtained powder using a
mixed acid of hydrofluoric acid, nitric acid and vitriolic acid, an
impurity analysis was performed using an ICP light emitting
analyzer (CIROS-120 produced by SPECTRO) and the contained amount
of each impurity (Li, B, Na, Mg, Al, P, K, Ca, Ti, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, Zr, Mo, Pd, Cd, Sb, Ba, W) was 1 ppm or less.
Particle size distribution was measure using a laser diffraction,
scattering method using a particle size distribution measurement
device and the average particle size was 449.7 .mu.m.
Next, an SEM image observation of the powder was performed using a
scanning electron microscope (SEM JSM-6390 produced by Nihon
Denshi). The particle diameter of primary particles which were
measured from the SEM image was 20 .mu.m or more and 180 .mu.m or
less.
The specific surface area was measured using a constant volume type
gas adsorption method using a specific surface area measurement
device (NOVA3000e produced by Sysmex) and calculated using a BET
multi-point method. Furthermore, the measurement sample was aerated
at 200.degree. C. for 3 hours in an N.sub.2 flow atmosphere in
advance. The specific surface area of the silicon carbide powder of
the Example obtained in this way was 0.15 m.sup.2/g. The properties
of the powder are shown in FIG. 1.
Example 2
Silicon carbide powder (15H2, average particle diameter 0.5 .mu.m
produced by Pacific Rundum Co., Ltd) was filled into a graphite
mold and after preforming, a silicon carbide sintered body was
obtained by heating under an argon atmosphere at a temperature of
2000.degree. C. for 2 hours at a pressure of 30 MPa using a high
frequency induction heating type hot press device. The density of
the obtained sintered body was measured using a dimensions, mass
measurement and was measured at 1.95 g/cm.sup.3 (absolute density
60.5%). A similar process as in Example 1 was performed on the
created silicon carbide sintered body to obtain a silicon carbide
powder. The obtained powder characteristics are shown in FIG. 1 the
same as Example 1.
Example 3
Silicon carbide powder (T-1, average particle diameter 0.03 .mu.m
produced by Sumitomo Osaka Cement) was filled into a 50 mm interior
diameter graphite mold and after preforming, a silicon carbide
sintered body was obtained by heating under an argon atmosphere at
a temperature of 2200.degree. C. for 2 hours at a pressure of 20
MPa using a high frequency induction heating type hot press device.
The density of the obtained sintered body was measured using a
dimensions, mass measurement and was measured at 1.37 g/cm.sup.3
(absolute density 42.4%). A similar process as in Example 1 was
performed on the created silicon carbide sintered body to obtain a
silicon carbide powder. The obtained powder characteristics are
shown in FIG. 1 the same as Example 1.
Example 4, Comparative Examples 1.about.3
Example 4 and Comparative Examples 1.about.3 were performed using
the same method as Example 1. Each condition of the samples,
sintering conditions, sintering characteristics and powder
characteristics are shown in FIG. 1 the same as in Example 1.
Comparative Examples 4
Apart from adding 0.5% by mass of boron carbide (HD20 average
particle size 0.5 .mu.m produced by H.C. Starck) as a sintered
auxiliary agent, the same processes were performed as in Example 2.
The results are shown in FIG. 1 the same as Example 1. The specific
surface area of the silicon carbide powder after crushing was below
a measurement minimum. (measurement minimum 0.01 m.sup.2/g) In
addition, 3400 ppm of boron was included as a result of an impurity
analysis using an ICP light emitting analyzer.
As is clear from FIG. 1, the silicon carbide powder in Examples
1.about.4 obtained by the method of the present invention has a
sufficient average particle diameter and specific surface area and
few contained impurities. However, the sintered body created
according to the conditions in Examples 3 and 4 had low strength
and the yield of the obtained silicon carbide powder was low. On
the other hand, a sintered body could not be created in Comparative
Examples 1.about.3 and impurities were contained in Comparative
Example 4.
(Nucleus Addition Method)
Example 5
1. Synthesis of a Raw Material Silicon Carbide
Metal silicon and acetylene black were weighed to produce a mol
ratio (C/Si) of elements of 1.0 between silicon and carbon, and the
raw material powder (raw material powder A) was adjusted after
mixing using a mortar in ethyl alcohol and dried. The raw material
powder A was put into a graphite crucible, and the silicon carbide
powder a was obtained by heating in a carbon heater under an argon
atmosphere at a temperature of 2000.degree. C. for 2 hours. The
average particle diameter of the obtained silicon carbide powder a
was 17.5 .mu.m. Furthermore, 10% by mass of the raw material
silicon carbide powder was added to the raw material powder A,
heating and sintering were performed by the process described above
and a raw material silicon carbide b was obtained. The average
particle diameter of the obtained raw material silicon carbide b
was 44.8 .mu.m.
2. Synthesis of Silicon Carbide Powder
Next, 10% by mass of the raw material silicon carbide b was added
with respect to the raw material powder A with a C/Si ratio of 1.0,
and the raw material powder (raw material powder B) was adjusted
after mixing using a mortar in ethyl alcohol and dried. The raw
material B was inserted into a graphite crucible and silicon
carbide powder was obtained by heating in a carbon heater under an
argon atmosphere at a temperature of 2000.degree. C. for 8
hours.
The obtained silicon carbide powder was separated by passing
through a sieve with 125 .mu.m gaps. The yield at 125 .mu.m or more
was 88%.
The average particle diameter of the obtained silicon carbide
powder was 163.2 .mu.m and the specific surface area was 0.26
m.sup.2/g. In addition, an SEM image observation was performed and
the particle diameter of primary particles which were measured from
the SEM image was 10 .mu.m or more and 40 .mu.m or less.
Furthermore, as a result of a crystal phase analysis, all of the
silicon carbides were .beta. types (3C phase) and an impurity
analysis was performed using an ICP light emitting analyzer and the
contained amount of each impurity was 1 ppm or less.
Examples 6.about.8, Comparative Examples 5.about.8
1. Synthesis of Raw Material Silicon Carbide
A raw material silicon carbide powder added in Examples 6.about.8
and Comparative Examples 5.about.8 was obtained by repeating the
processes by the same method as in Example 5 a plurality of times.
The heating temperature, the number of times each process was
repeated and characteristics of the raw material silicon carbide
powder are shown in FIG. 2.
2. Synthesis of Silicon Carbide Powder
Apart from the average particle size and added amounts of the raw
material silicon carbide powder added shown in FIG. 2, the powder
was created by the same method as in Example 5. The heating
temperature and characteristics of the obtained silicon carbide
powder are shown in FIG. 2.
As is clear from FIG. 2, the silicon carbide powder in Examples
5.about.8 obtained by the method of the present invention has a
sufficient average particle diameter and specific surface area.
However, in Example 8, it was clear that the powder characteristics
did not show large effects compared to the added raw material
silicon carbide powder. However, the silicon carbide powder in
Comparative Examples 5.about.8 had a small particle diameter.
(Solid-Phase Reaction Method)
Example 9
45% by mass of a metal silicon (purity 5N produced by Kojundo
Chemical Co) with an average particle diameter of 308.2 .mu.m and
55% by mass of a resol type phenol resin (J-325 produced by DIC)
were mixed in ethyl alcohol and after removing the solvent a raw
material mixed product was obtained by heat curing at 130.degree.
C. This was carbonized by heating for 1 hour at 1000.degree. C.
under an argon atmosphere. The obtained carbide raw material mixed
product was analyzed for elements using an Igniting--IR method
(IR-412 Produced by LECO) and a dehydration weight ICP light
emission analysis combined method (CIROS-120, compliant to JIS 1616
produced by SPECTRO) and the C/Si was 1.12.
The obtained carbide raw material mixed product was put into a
graphite crucible and a silicon carbide powder was obtained by
heating for 10 hours at 1400.degree. C. under an argon atmosphere.
As a result of a crystal phase analysis of the obtained silicon
carbide, there was no non-reacted silicon or carbon and only .beta.
type (3C phase) silicon carbon. In addition, the average particle
diameter was 289.4 .mu.m.
Furthermore, the obtained silicon carbide powder was put into a
graphite crucible and heated for 2 hours at 2000.degree. C. under
an argon atmosphere in a carbon heater furnace. The average
particle diameter of the heated powder was 284.1 .mu.m and the
specific surface area was 0.21 m.sup.2/g. In addition, an SEM image
observation was performed and the particle diameter of primary
particles which were measured from the SEM image was 8 .mu.m or
more and 50 .mu.m or less. Furthermore, an impurity analysis was
performed using an ICP light emitting analyzer and the contained
amount of each impurity was 1 ppm or less.
Example 10, Comparative Examples 9.about.13
Example 10 and Comparative Examples 9.about.13 were performed using
the same method as Example 9. Each condition of the raw material in
the sintering process of the silicon carbide, sintering conditions
and post process sintering conditions and characteristics of the
obtained silicon carbide powder are shown in FIG. 3.
Comparative Examples 14
Apart from using carbon black as the carbon source, the powder was
created using the same method as Example 9. The characteristics of
the obtained silicon carbide powder are shown in FIG. 3.
As is clear from FIG. 3, the silicon carbide powder in Examples 9
and 10 obtained by the method of the present invention has a
sufficient average particle diameter and specific surface area.
However, in the Comparative Examples 9, 12 and 14, the average
particle diameter of the silicon carbide powder after the silicon
carbide sintering process was small and the particle was also
insufficient after post processing. In addition, in Comparative
Example 13, the primary particle diameter was small and sintering
between particles was weak making the powder easy to fall apart. In
Comparative Example 10, it was clear that non-reacted silicon
remained and in Comparative Example 11 silicon carbide was not
produced.
Creation of Silicon Carbon Single Crystal
Example 11
5 g of the silicon carbide powder created in Example 1 was filled
into a graphite crucible with an 11 inch interior diameter. Next, a
4H--SiC (0001) single crystal plate was placed on a lid section as
a seed crystal. The graphite crucible was placed into a high
frequency induction heating heater and after sufficiently
performing argon replacement, a single crystal was cultivated with
an atmosphere pressure of 10 Torr and the bottom surface
temperature of the crucible container set at 2000.degree. C. The
sublimation rate after 2 hours of growth and after 20 hours of
growth and length of the single crystal after 20 hours of growth
are shown in FIG. 4. Here, the sublimation rate is a value
calculated by dividing the reduced amount (sublimation amount) of
the raw material silicon carbide powder after heating by the growth
time period.
Comparative Example 15
A single crystal was grown using the same method as in Example 11
using a commercially available high grade purity silicon carbide
powder (GMF-CVD produced by Pacific Rundum Co., Ltd). The average
particle diameter of the raw material silicon carbide was 682.9
.mu.m and the specific surface area was below a minimum
measurement. (below 0.01 m.sup.2/g) The results are shown in FIG. 4
the same as Example 11.
Comparative Example 16
A single crystal was grown using the same method as in Example 11
using a commercially available polishing silicon carbide powder
(NC-F8 produced by Pacific Rundum Co., Ltd). The average particle
diameter of the raw material silicon carbide was 2350 .mu.m and the
specific surface area was below a minimum measurement. (below 0.01
m.sup.2/g) The results are shown in FIG. 4 the same as Example
11.
Comparative Example 17
A single crystal was grown using the same method as in Example 11
using a high grade purity silicon carbide powder (15H2 produced by
Pacific Rundum Co., Ltd). The average particle diameter of the raw
material silicon carbide was 0.5 .mu.m and the specific surface
area was 16.1 m.sup.2/g. The results are shown in FIG. 4 the same
as Example 11.
Examples 12.about.16, Comparative Examples 18.about.20
A single crystal was grown using the same method as in Example 11
using the silicon carbide powder created in the Examples 2, 5, 6, 9
and 10, and Comparative Examples 5, 6 and 9. The results of the raw
material characteristics and after single crystal growth are shown
in FIG. 4 the same as Example 11.
As is clear from FIG. 4, in the Examples 11.about.16 using the
silicon carbide powder in Examples 1, 2, 5, 6, 9 and 10 obtained
using the method of the present invention, the raw material silicon
carbide was sublimated at a stable rate and a single crystal was
grown without a reduction in the sublimation rate over 20 hours
compared to a sublimation rate over a growth period of 2 hours.
However, in Comparative example 15, although a raw material silicon
carbide was sublimated at a stable rate, the raw material
sublimation amount was less than the silicon carbide powder created
using the method of the present invention. In addition, in
Comparative Example 16 which used a raw material silicon carbide
with a large particle diameter, the amount of sublimation was small
and the single crystal growth was 1 mm or less. In the Comparative
Examples 17 which used a raw material silicon carbide with a small
particle diameter and the Comparative Examples 18.about.20 which
used the silicon carbide powder in Comparative Examples 5, 6 and 9,
it was clear that the sublimation rate decreased after 20 hours of
crystal growth.
The silicon carbide powder for the production of a silicon carbide
single crystal of the present invention displays a high and stable
sublimation rate during single crystal growth.
Industrial Applicability
The silicon carbide powder of the present invention displays a high
and stable sublimation rate during a sublimation recrystallization
method and can be favorably used as a raw material for a silicon
carbide single crystal.
* * * * *